Fiber-optic current transducers (FOCTs) can be used to detect alternating currents in transmission lines. FOCTs operate based on the principle of Faraday rotation, which is a magneto-optical effect whereby a rotation of the polarization plane of a light beam confined in a fiber-optic waveguide placed near the transmission line occurs in response to a magnetic field induced by the current in the transmission line. The angle of the rotation is linearly proportional to the component of the magnetic field in the direction of light propagation in the waveguide. As such, a change in angle can be correlated with the strength of the magnetic field, which can in turn be used to calculate the current in the transmission line.
An FOCT can be used to detect an alternating current using a differential current measurement configuration. In such a measurement scheme, one or more conductors pass through a common fiber-optic loop of the FOCT. This configuration only works if the current in the conductors algebraically add to zero. For example, for a normal 3-phase transmission system, the currents are 120 degrees out of phase from one another, and they algebraically add to zero. Therefore, the current in the transmission system can be detected by the FOCT, simply by monitoring a differential current in the transmission system. Similarly, for a single phase transmission system, currents in the conductors are 180 degrees out of phase, and they add up to zero.
In differential current measurements, improper alignment between a polarizer unit and a mirror unit of the fiber-optic loop can lead to large phase and amplitude root-mean-squared (RMS) errors in the measured differential current. As such, since differential current measurements involve currents in the milli-Ampère (mA) regime, the accuracy of a current reading can suffer largely as a result of improper alignment between the polarizer unit and the mirror unit.